Multinuclear MAS NMR Identification of Fluorine Species on the Surface of Fluorinated γ-Alumina

Abstract

The fluorination on the γ-alumina surface was studied by multinuclear MAS NMR and BET measurements. The fluorine species on the fluorinated γ-alumina can be identified by 19F MAS NMR combined with 1H MAS NMR and 27Al MAS and CP/MAS NMR. As proven by 1H MAS NMR, fluorination of the alumina preferentially replaced the basic hydroxyl groups. Quantitative analysis showed that, at lower fluoride loadings, one alumina hydroxyl is substituted by one fluoride without breaking the bridging Al−O−Al bonds. At high fluoride content, more of the acidic hydroxyl groups are displaced by fluorine, and the number of added fluorine atoms exceeds the number of lost hydroxyl groups. Bridging Al−O−Al bonds are broken at high fluoride loading, and isolated AlxFy·nH2O fractions appear as observed by 19F MAS and 27Al MAS NMR. Three types of fluorine species were detected in the 19F MAS NMR spectra of the fluorinated γ-alumina, which could be assigned to the fluorine species bound to one, two, and three octahedral aluminum atoms.